The present invention relates to the field of assembling transfer layers and substrates. In particular, it is applicable to the production of light-emitting diodes (LEDs) or laser diodes (LDs) using layer transfer by metal bonding.
The active layers of LEDs and LDs emitting in green, blue or ultraviolet (constituted, for example, by GaN or AlGaInN alloys) are usually grown epitaxially on sapphire or SiC substrates (see, for example, IEEE Journal On Selected Topics in Quantum Electronics, 8 (2002), 264, T. Mukai, “Recent progress in group-III nitride light emitting diodes”, or Phys. Stat. Sol. (a) 180 (2000) 5, V. Härle et al., “GaN-based LEDs and lasers on SiC”), while the active layers of LEDs or LDs emitting in the red, orange or yellow (constituted, for example, by GaAs or AlGaInP alloys) are essentially grown epitaxially on GaAs or InP substrates. (See, for example, IEEE Journal on Selected Topics in Quantum Electronics 8 (2002) 321, K. Streubel et al., “High brightness AlGaInP light emitting diodes”). These substrates are typically bulk substrates and are selected principally for their physical properties, such as their lattice parameters and thermal coefficient of expansion suitable for the growth of active layers by epitaxy.
However, substrates such as silicon or certain metals would be preferred for the operation of the final device because of better thermal conductivity (dissipation of heat produced by LEDs or LDs in operation) and better electrical conductivity (for example, for establishing good electrical contact with the back face of the device, which is positioned opposite to the face of the device that is used to extract emitted light) provided by such substrates.
One known way of benefiting from those two types of substrates and their combined advantages consists of growing the active layers of the device on a first type of substrate (e.g., sapphire, SiC or GaAs) and transferring that prefabricated device, by metal bonding, to a substrate adapted for the final steps of its fabrication and, in particular, to its optimum function. The transfer can be divided into two critical steps involving metal bonding of a final support to the active layers epitaxially grown on the initial substrate, and removal of the initial substrate.
One known technique for the removal of the initial support is a laser lift-off separation technique, which is described in U.S. Pat. No. 6,071,795. That technique consists of using a laser to disintegrate the interface separating the initial substrate from a first thin layer reposing on the initial substrate. The laser is used to illuminate the starting structure from its back face. The disintegrated interface releases the thin layer from the initial support. This technique is limited to starting structures comprising an initial surface that is transparent to the laser beam and a first thin layer that absorbs the laser light. The only application of this technique appears to be that of forming a thin layer of GaN epitaxially grown on sapphire.
U.S. Pat. No. 6,335,263, which is incorporated herein by reference in its entirety, discloses the use of a stack based on In and Pd to bond an LED structure based on GaN to a Si or GaAs substrate at 200° C. The starting sapphire substrate is removed by laser separation. The Pd—In stack is unsuitable for mechanical release of the initial support substrate because of its poor mechanical strength. Further, the PdIn3 layer formed does not constitute a good mirror for LED light.
Other known means for removing the initial support consist of mechanical thinning (grinding or lapping) or chemical etching or a combination of mechanical thinning with chemical etching. These methods remove the initial support to transfer the active layer of a particular device, but also destroy the initial support at the same time.
Mechanical thinning can be particularly difficult with fragile substrates such as GaAs, and is lengthy in duration for hard substrates such as SiC, GaN, AlN or sapphire. Chemical attack is also lengthy in duration and is limited to substrates for which a solution (an acid) exists which can selectively etch the support and not the active layers epitaxially grown thereon. GaAs and Si are almost the only examples of an initial support that falls into this category. Selective chemical etching of GaN, SiC, AlN and sapphire presently appears to be impossible.
As an example, the K. Streubel et al. article mentioned above discloses the use of an AuSn alloy to carry out metal bonding at 350° C. of GaAs-based LED structure to a Si substrate. The initial GaAs substrate, in many cases subsequently chemically etched, is not recyclable because it is destroyed during the process.
In methods for fabricating LEDs developed by the Visual Photonics Epitaxy Co (which, for example, are described in U.S. Pat. No. 6,287,882), an AuBe alloy is used for metal bonding. This method is also uneconomical because the initial substrate is removed by chemical etching during the processing technique, and is therefore, not available for recycling.
Another known technique is shown in PCT Publication No. WO 02/33760, which is incorporated herein by reference in its entirety. This PCT publication shows a method of producing LEDs on a composite substrate. In this technique, releasing methods include chemical etching (destruction of the initial support), which is suitable for GaAs and Si but not for SiC, GaN or sapphire, and laser separation, which is limited to a transparent substrate such as sapphire and is unsuitable for Si and SiC supports.
In the method described in that document, motifs are produced in the active layers prior to application of the final support. The particular processing step of producing motifs by a combination of photolithography and etching is complex, expensive and renders the structure ensemble weak. In such techniques, the motifs result in establishing prime rupture zones. Solutions given in WO 02/33760 are thus not universal (e.g., not a solution for SiC supports), and not inexpensive because recycling can only be envisaged with a sapphire support that is removed by laser separation. In addition, mechanical release is excluded by the type of structure proposed in WO 02/33760 because the motifs render the structure of the active layers too weak for that treatment.
Further, that document also discloses the presence of a metallic mirror layer to ensure electrical contact, and it may create deficiencies through the diffusion of metals into the active layers of the LED.
A further known means for removing an initial substrate consists of detaching or mechanically releasing a structure produced by epitaxial growth on a substrate, e.g., a substrate formed from a support and a thin layer that weakly adheres to the support. Several types of mechanically releasable substrates can be envisaged. Such techniques are, for example, described in PCT publication WO 02/084622 and US published application no. US 2003/077885, both of which are expressly incorporated herein in their entireties by reference thereto.
That approach, necessitating the use of a releasable initial substrate, is relatively universal with respect to the nature of the initial substrate and also allows the initial support to be recycled after release. This approach is not limited to transparent substrates (as is the case with laser separation) or to materials that can be chemically etched. Once the active layers are separated from the initial support, the initial support can be recycled and used again as a releasable substrate for epitaxial growth. However, the stresses inherent to mechanical release of the starting substrate require a strong bond between the active layers and the final substrate to prevent a fracture from occurring.
Therefore, a problem exists with respect to providing a method of producing a thin layer on a final substrate from a starting substrate which allows the latter to be recycled (regardless of the nature), by dint of non-destructive mechanical release.
Regarding the device itself, there is currently no known combination of layers between the final substrate and the active layers having for example the properties of ohmic contact, optical reflectivity and optionally electrical conductivity and/or mechanical strength. The present invention now seeks to overcome these problems.